The present invention relates to a process for preparing 5-deoxy-L-arabinose. 5-Deoxy-L-arabinose is useful as a starting material for preparing 5,6,7,8-tetrahydro-L-biopterin being useful for treatment of patients with Parkinson's disease or depression as well as phenylketone urea.
A process employing L-rhamnose as a starting material and another process employing L-arabinose as a starting material have been known for preparing 5-deoxy-L-arabinose. However, it is not preferable to employ L-rhamnose in large amount in industry since L-rhamnose is costly.
As a process employing L-arabinose which is easily obtained in industry as a starting material, there has been known a process where tosylated L-arabinose-diethylmercaptal is reduced with lithium aluminum hydride (hereinafter referred to as "LiAlH.sub.4 ") to give 5-deoxy-L-arabinose-diethylmercaptal and then the obtained 5-deoxy-L-arabinose-diethylmercaptal is converted to 5-deoxy-methyl-L-arabinofuranoside by defulfurization and O-methylation, and further the obtained 5-deoxy-methyl-L-arabinofuranoside is reacted with hydrochloric acid to give 5-deoxy-L-arabinose as reported by B. Green and H. Rembold in "Chem. Berichte", 99, 2162 (1966). However, the above process is not preferable for industrially preparing 5-deoxy-L-arabinose because LiAlH.sub.4 which is costly and dangerous must be employed as a reductant and mercuric chloride (hereinafter referred to as "HgCl.sub.2 ") which has associated problems in industrial usage must be employed in desulfurization.
And also, a process by which 5-deoxy-D-arabinose is obtained from 5-tosyl-D-arabinose-diethylmercaptal via 5-deoxy-D-arabinose-diethylmercaptal has been disclosed by H. Zinner et al. in "Chem. Berichte", 92, 1618 to 1623 (1959). However, this process also employed LiAlH.sub.4 and HgCl.sub.2 as in the case of the process disclosed by Green et al. That is, deoxygenation reaction in this process is carried out in the presence of LiAlH.sub.4 in a mixed solvent of benzene and ethyl ether. However, the desired compound in this reaction is obtained only in less than 60% yield. Successive desulfurization is carried out in acetone containing water in the presence of mercury oxide and HgCl.sub.2 and the recovery of mercury is carried out by passing hydrogen sulfide through the resultant. However, the yield is less than 50% and moreover it is very difficult to recover mercury completely. Therefore, this process is not industrially suitable.
In the process for preparing 5-deoxy-L-arabinose, the present inventors have found that sodium boron hydride (hereinafter referred to as "NaBH.sub.4 ") which is cheap and quite safe can be employed as a reductant and desulfurization of 5-deoxy-L-arabinose-dialkylmercaptal can be carried out in the reaction system of di-methylsulfoxide (hereinafter referred to as "DMSO")hydrochloric acid to give a high yield without employing mercury, and the like.
An object of the present invention is to provide a process for preparing 5-deoxy-L-arabinose from 5-tosyl-L-arabinose-dialkylmercaptal in a high yield without employing a heavy metal such as mercury.